A "current mirror" circuit is one simple form of current source that is typically implemented as an integrated circuit. A current mirror circuit employing bipolar transistors suffers, however, from the disadvantage of providing an output current whose magnitude is uncertain and remains substantially constant over only a relatively narrow range of operating temperatures. The reason is that the output current depends on the base-to-emitter voltage of at least one transistor used in the current mirror circuit whose voltage varies with different integrated circuits and changes in temperature. A current source of this type is, therefore, undesirable for use in applications that require an output current of predictable magnitude or an output current whose magnitude remains constant over a wide range of operating temperatures.
A current source of conventional design which includes a special "bias" operational amplifier provides an output current of constant magnitude that is independent of temperature. This type of current source is undesirable because the operational amplifier employs many transistor devices, which constitute a circuit of complex design that is difficult to implement with a single power supply, particularly at a relatively low voltage (e.g., +5 volts).